(57) Pseudo-random number (PN) sequence generators used in scrambling and descrambling
circuits at separate locations are periodically preset to the same start value in
order to maintain synchronism, with the preset value being also periodically changed.
The preset value can be transmitted to the scrambling and descrambling circuits over
a privacy channel once per frame and stored in storage registers (20) associated with
each of the scrambling and descrambling circuits (22, 24). The stored starting state
values can be loaded into the PN sequence generators (22) as often as desired to maintain
proper synchronization, with the stored values themselves being changed at a rate
dependent on the desired degree of security.
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